This looks like something straight from Dr. Seuss, especially in the trees that have a lot of them.
This is from a different tree, but might be the same thing. Certainly must be related, in any case - equally Seuss-like.
Here are three more things in the category of "strange things attached to trees." The middle one I thought might be some sort of strange nest. For that matter, the first one is rather round... I suppose it could be, too, though I've never seen one that color. I saw a number of things looking like the last one.
Anyone have any ideas? I have bird books to consult to identify the punk ducks I also saw on the walk (another blog post?), but strangely enough, we have no Peterson's Guide to Odd Things in Trees.